Composition and method for slowly dissolving siliceous material

ABSTRACT

An acidizing composition and method for slowly dissolving siliceous material by the slow production of very low concentrations of hydrofluoric acid is provided by this invention. The continual generation of very low concentrations of hydrofluoric acid is accomplished by the hydrolysis of hexafluorotitanate-containing compounds. The acidizing composition is particularly useful and advantageous for the solubilization of siliceous clay in or adjacent the pores of subterranean hydrocarbon formations thereby increasing the permeability of said formations. Because of the very low concentration of hydrofluoric acid in the acidizing composition solubilization of siliceous clay deposits is possible at relatively large distances from the point of injection into the formation, for acidizing formations which are too sensitive for more reactive acidizing compositions, for acidizing formations containing chlorite clays, and/or for acidizing formations having an elevated temperature including formation temperatures of 125° C. or higher.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to the stimulation of wells to improve thepermeability of such wells to the flow of fluids. The invention isespecially useful in improving the flow of hydrocarbons from wells whichhave suffered from formation damage due to clay deposits.

2. Description of the Prior Art

It is well known that oil production in siliceous subterraneanformations, over the useful life of a well, usually decreases with time.To reestablish a higher flow of oil, one of the first methods usuallyemployed is pumping. Frequently, however, after a period of time, evenpumping will not make the well economical. Unfortunately in many wellssuch flow reduction occurs long before the oil, or other fluid in thereservoir reached by the wellbore, has become depleted. Low permeabilityfrequently results from the deposition of clay and other finely dividedmaterial in the pore structure or flow passages of the formation. Clayparticles, capable of forming such undesirable deposits, generally existthroughout the formation and are carried by the oil and deposited in theflow passages leading to the wellbore. Formation damage can also becaused by the swelling of the clay upon contact with foreign liquidsinjected for well development or stimulation purposes. Formation damageof the above types is often referred to as clay deposits, claydispersions, particle plugging, clay swelling, etc., which hereinafterwill simply be referred to collectively as "pore deposits."

It is known that pore deposits can be solubilized more or less bytreatment with mineral acid solutions, for example, hydrochloric acidand hydrofluoric acid. Aqueous solutions containing about 2 to 6 weightpercent hydrofluoric acid and 5 to 15 weight percent hydrochloric acid,sometimes referred to as "mud acids" have been used to treat damagedformations in hopes of restoring to the formation its initialpermeability. Mud acids have also been used to treat formations whichare naturally tight.

Unfortunately, hydrochloric acid is usually not effective insolubilizing the more tenuous pore deposits such as those deposits thatare mainly siliceous in composition. By the term "siliceous" as usedherein is meant silica and/or silicate. By the term "siliceous material"as used herein is meant silica-containing and/or silicate-containingmaterials. Examples of siliceous materials are sandstone and certainclays. Non-limiting example of clays which are silicates, usuallyaluminosilicates are attapulgite, bentonite, chlorite, halloysite,illite, kaolinite, montmorillonite, and various mixtures of theaforementioned substances. It is known that hydrofluoric acid willsolubilize siliceous material readily; however, because of its highreactivity hydrofluoric acid, unmixed with other mineral acids such ashydrochloric acid, generally is not used to increase oil production.Other serious problems also exist with the use of hydrofluoric acid. Forexample, since the rate of reaction of hydrofluoric acid with siliceousmaterials is very rapid, most of the acid is spent within a zone ofabout one or two feet or less radially from the wellbore. In formationshaving high formation temperatures the acid becomes spent at evenshorter distances from the wellbore thereby causing the acidizingoperation to be even less effective.

Since the mineral content of the matrix of many formations is usuallysandstone or silica or a similar siliceous material, hydrofluoric acidcan dissolve the matrix itself as well as the undesirable pore depositsin the matrix. As a consequence hydrofluoric acid can cause permanentdamage to the formation by the dissolving of the pore structure ormatrix itself, or by allowing the precipitation of reaction productsand/or creation of fines within the pores of the formation. To preventpermanent damage from occurring the concentration of hydrofluoric acidis usually adjusted so that no more than minor damage to the formationcan occur. As a consequence, clay deposits distant from the wellbore donot come in contact with hydrofluoric acid-containing acidizingcompositions generally used for dissolving siliceous matter and thussuch distant deposits are not dissolved. It is well recognized in theoil producing industry that it is difficult to dissolve only the poredeposits and especially those deposits more than two feet from thewellbore. Nonetheless even utilizing hydrofluoric acid concentrations aslow as 0.1%, permanent damage to some formations can occur.

As mentioned earlier another problem associated with acidizing withformations containing hydrofluoric acid is that since the cleaned-outarea is usually within a two-foot radius or less of the wellbore, lossof permeability can reoccur within a very short period of time after thetreated well is put back on production since the deeper pore depositsare not removed. Thus, it is generally accepted that if deep poredeposits are to be solubilized by hydrofluoric acid, a large quantityand flow rate of acid must be used and since the acid can react with allsiliceous material, there is a very high risk that permanent damage willresult to the formation.

For these reasons, there have been various attempts to slow up the rateof reaction of hydrofluoric acid so that it can penetrate deeper intothe formation and solubilize deeper pore deposits without causingserious damage to the formation adjacent to the wellbore. Unfortunately,many of these attempts, as will be described and further discussedbelow, still fall short of effectively increasing the permeability ofthe formation in those zones much farther than the usual two feet fromthe wellbore.

U.S. Pat. No. 990,969 discloses a well stimulation process whichproduces hydrofluoric acid directly in the subterranean formation. Inparticular, a quantity of hydrochloric acid is first pumped into thewell which is then followed by a quantity of sodium fluoride. Thehydrochloric acid and the sodium fluoride react in the formation toproduce hydrofluoric acid and sodium chloride. The hydrofluoric acidreacts with the silica material in the pores to dissolve it therebyincreasing the permeability of the formation to the flow of oil. Thepatentee alleges that in his process the hydrofluoric acid is notrequired to be handled at the surface or continuously in the tubing ofthe wellbore, but rather produced from precursor reagents within thesubterranean formation itself. The "alternate and separate slug" or "twoslug" method as disclosed in U.S. Pat. No. 1,990,969 also hasdisadvantages. First damage to the wellbore can occur because at theinterface of the alternate slugs, hydrofluoric acid can be generated andcan attack the well tubing itself, thereby decreasing the useful life ofsuch tubing. Secondly, mixing of the reactants within the porousformation is not always uniform and hence not always complete, therebycausing some regions to have high hydrochloric acid concentrations andother regions to have high sodium fluoride concentration. Such regionswill not be exposed to effective hydrofluoric acid concentrations andconsequently will be largely unaffected by the treatment. Thirdly, thepermeability tends to be increased only in a region very close to thewellbore due to the high reactivity of the treating solution.Consequently, deep pore deposits will not be solubilized to the extentdesired. Thus, any improvement in oil production will be most likely foronly a relatively small period of time.

Others have attempted to improve the distribution of the reactants intothe formation over a greater distance by slowing the rate of reactionbetween hydrofluoric acid and the formation. U.S. Pat. No. 3,889,753discloses a well stimulation method for dissolving silica or clay aroundthe wellbore. The method involves contacting the siliceous material withan aqueous solution of a fluoride salt, a weak acid, and a weak acidsalt in proportions that form in situ a significant but lowconcentration of hydrogen fluoride. However, while such acidizingmixtures may provide some improvement they still are too reactive toreach and solubilize deep pore deposits.

Ostensibly in order to overcome this difficulty some have returned tothe alternate but separate slugs approach discussed earlier. U.S. Pat.No. 4,056,146 discloses a well stimulation method in which alternateslugs of hydrochloric acid and ammonium bifluoride or ammonium fluoride,or mixtures thereof, are alternately and separately introduced into thewellbore. The reagents react and produce hydrofluoric acid.Unfortunately these reagents still react too fast and the hydrofluoricacid produced is spent before it can penetrate deeply into theformation.

In order to obtain deeper penetration of the reagents into theformation, U.S. Pat. No. 4,136,739 varied the injection sequence byinjecting, between the two alternate and separate reagent slugs, ahydrocarbon liquid such as diesel oil. In particular, an aqueoussolution of an ammonium salt of hydrofluoric acid such as ammoniumfluoride is injected in a first slug into the formation. This is thenfollowed by a separate slug of diesel oil, which in turn is followed bya third and separate slug of hydrochloric acid. The patentee contendsthat in this way hydrofluoric acid is generated at a deeper distancefrom the wellbore than with the usual two slug method. The problem withthis method is that mixing of the reagents in situ, because of theinterdisposed diesel oil, becomes even more difficult and lesseffective. Furthermore, the reactants, when they are mixed, reactquickly and produce a relative high concentration of hydrofluoric acidwhich is too reactive to penetrate deeply into the formation.

In many of the alternate and separate slug methods, the steps arerepeated a number of times in order to better distribute the reagents ona more uniform basis into the formation. This switching back and forthcan lead to operator error which in turn can result in regions in theformation having a higher concentration of one reagent and little, ifany, of the other reagent, thereby providing no solubilization of thepore deposits in such regions. Unfortunately, the difficulty with thealternate and separate slug methods is that it is difficult to providean equal distribution of each reagent to all parts of the formation zoneunless the amount of each slug is very small. As can be appreciated, thesmaller the slug amount the greater the number of slug cycles requiredto introduce the required quantity of reagents into the formation. Asslug amount is decreased and the number of cycles increased, the moreapt the reagents are to react and form hydrofluoric acid beforepenetrating deeply into the formation thereby increasing the possibilityof both formation damage and well casing damage, and producing little,if any, solubilization of the deeper pore deposits.

The difficulty of mixing reagents in situ was avoided in U.S. Pat. No.418,118 by mixing the acidizing composition at the surface prior toinjecting into the formation. The reaction rate of hydrofluoric acid onsilica and silicates is said to be retarded. The method relies on thereaction of a mineral acid other than hydrofluoric acid with certainfluoride compounds to produce hydrofluoric acid. The fluoride compoundsdisclosed have the formula:

    (T.sup.+3).sub.n (M.sup.+1).sub.z (F.sup.a).sub.y

and include their hydrates. The cation T is zirconium, cobalt orchromium. M is either hydrogen or ammonium, and z is 0 to 4. Theconstants satisfy the formula:

    3n+z=ay.

The only fluoride compounds disclosed are chromium fluoride, cobaltfluoride, ammonium zirconium hexafluoride or (NH₄)₂ ZrF₆, and hydrogenzirconium hexafluoride or H₂ Zr₄ F₆. In order to produce hydrofluoricacid it is taught that sufficient mineral acid, other than hydrofluoricacid, is required to produce an acidic composition with a pH no greaterthan 2. It is further taught that the actual pH is ordinarily much lessthan 2 and is often expressed in negative values. It is stressed thatthe only real limitation on the operability with respect to aciditycaused by the mineral acid is the upper pH limit of 2 and that this canbe achieved with an acid (ostensibly a mineral acid other thanhydrofluoric acid) concentration of about 0.1 percent acid by weight ofacidic composition. This method has the disadvantage of requiring areaction between a mineral acid and a fluoride compound to producehydrofluoric acid while requiring a strongly acidic solution since theupper limit of the pH is 2 and ostensibly in actual practice a pH muchless than 2 or even negative values must be utilized if the treatment isto have any real effect on increasing the permeability of the formation.

Another known method depends upon the hydrolysis of fluoboric acid(HBF₄) to produce hydrofluoric acid in situ in the formation. While someimprovement in dissolving deeper pore deposits may occur in somesubterranean formations the reaction rate is still too high for the moresensitive formations; see Journal of Petroleum Technology, August 1981,pages 1491 to 1500.

In all of these prior art methods, the reactants are still ostensiblytoo reactive to penetrate deeply into the formation and solubilize thedeeper pore deposits. Accordingly, there remains a need for a processwhich retards the rate of reaction of hydrofluoric acid in the formationbut at the same time provides sufficient hydrofluoric acid to thevarious parts of the formation without a dependence on mixing of thereagents within the formation as the hydrofluoric acid is consumed. Whatis therefore needed is to have a very small amount of the reactant,hydrofluoric acid, present at all times without the need to rely on itsformation in situ by the mixing of alternate and separate slugs ofprecursors, or even by the mixing within a single slug of reagents theinterreaction of which might be altered by mineral matter and/or brinein the formation. In general, it is believed that the prior artacidizing solutions utilizing hydrofluoric acid have too high areactivity and hydrofluoric acid concentration to effect solubilizationof the deeper pore deposits. The present invention offers a solution tothese problems by the very slow in situ formation of very small amountsof hydrofluoric acid, by hydroysis of a fluoride compound without thenecessity to react the fluoride compound with a mineral acid or anyother reagent thereby minimizing the uncontrollable effect of varyingmineral matter and brine encountered in the subterranean formation beingacidized. The present invention therefore allows deeper penetration ofthe treating fluid into the formation to solubilize deeper pore depositswithout causing significant damage to the matrix structure of theformation.

SUMMARY OF THE INVENTION

Injection of one or a series of acidic solutions down a well and into asubterranean formation with the objective of improving the permeabilityof the formation and hence the flow rate of fluid, for examplepetroleum, natural gas, water, or other fluids into or out of the wellis commonly termed matrix acid stimulation. Frequently concentratedsolutions, e.g., 5 to 30% hydrochloric acid, or mixtures such as 12%hydrochloric acid 3% hydrofluoric acid termed mud acid are used tostimulate the formation. Unfortunately the use of these concentratedacid solutions can cause reduction in the permeability of certainsubterranean formations for a variety of reasons, including very rapidreaction with the formation leading to reaction product precipitationand release of fine particles. For such sensitive formations, whichinclude high clay containing rock, a more gentle acidizing agent isrequired.

This invention comprises the slow hydrolysis of hexafluorotitanate anionor TiF₆ ⁻⁻, to produce low concentrations of hydrofluoric acid (HF)which is then used to slowly dissolve siliceous matter which restrictsthe flow of fluids in the formation. This invention also comprises theslow hydrolysis of hexafluorotitanate anion-containing compound withoutthe necessity of reacting the hexafluorotitanate with an acid, includingan acid other than HF, to produce HF. The equilibrium constant for thehydrolysis of hexafluorotitanate anion to HF is about 1.4×10⁻⁶, seeKinetics of the Hydrolysis of the Hexafluorotitanate Ion in AqueousSolution of 0° and 25° C., Russian J. Phy. Chem., Vol. 46, pages1334-1336, which is hereby incorporated herein by reference. Thereforethe concentration of HF in solution at equilibrium is very small. Thekinetics of the hydrolysis of hexafluorotitanate anion are also slowabout 1 to 2 hours at 25° C. This invention comprises the reaction of anaqueous solution containing hexafluorotitanate anions with siliceousmaterials to slowly dissolve such materials. When subterraneanformations containing siliceous materials are contacted with theacidizing solutions of this invention the very slowly producedhydrofluoric acid, because of its very low concentration, will reactslowly with the formation, and preferentially with the more reactivesiliceous clay deposits in or adjacent the pores of the formation. Theacidizing solution therefore will travel deeply into the formationthereby improving the permeability of the formation for considerablylonger distances from the wellbore than could be achieved by moreconcentrated hydrofluoric acid solutions.

Accordingly, in accordance with the practice of the present invention,there is provided a composition and method for slowly dissolvingsiliceous material comprising forming an aqueous acidizing compositioncomprising (i.) water and (ii.) a first substance selected from thegroup consisting of hexa-fluorotitanate-containing compounds, hydratesof such compounds, and mixtures thereof which are operable for producingwithout the presence of an acid in said aqueous composition other thanhydrofluoric acid, hydrofluoric acid by reaction or hydrolysis ofhexafluorotitanate anions produced from the first substance with water,wherein the concentration of the first substance in the aqueouscomposition is from about 0.0001 molar to about the solubility limit ofthe first substance in water. The method further comprises contactingsiliceous material with the aqueous composition to slowly dissolve thesiliceous material. The method is particularly useful where thesiliceous material is contained in a subterranean formation, andespecially where the siliceous material is siliceous mineral matteradjacent to the walls of, or in, the pores of the subterraneanformation. In one embodiment the first substance is selected from thegroup consisting of ammonium fluotitanate or (NH₄)₂ TiF₆, sodiumfluotitanate or Na₂ TiF₆, hydrates of ammonium fluotitanate, hydrates ofsodium fluotitanate, and mixtures thereof. The first substancehydrolyzes in the aqueous acidizing composition to produce thehexafluorotitanate anion which then hydrolyzes by the followingreaction:

    TiF.sub.6.sup.-- +H.sub.2 O=2HF+TiOF.sub.4.sup.--

In one embodiment the concentration of the first substance used to formthe aqueous acidizing composition is from about 0.01 to about 1.3 molarand has a pH greater than 2. In another embodiment the concentration ofthe first substance used to form the aqueous acidizing composition isfrom about 0.05 to about 0.75 molar and has a pH of at least about 2.2.In still another embodiment the concentration of the first substance isfrom about 0.1 molar to about 0.5 molar. In still another embodiment thepH of the aqueous solution is about 2.6 or higher. In yet anotherembodiment the pH of the aqueous solution is from about 2.2 to about3.8. In one embodiment the pH of the aqueous solution is from about 2.6to about 3.7. In another embodiment the pH of the aqueous solution isfrom about 2.6 to about 3.5.

In a subterranean formation which contains siliceous materials, usuallyboth the pore deposits and the matrix of the formation contain siliceoussubstances. Fortunately the siliceous material which comprises the poredeposits is usually more easily solubilized than the siliceous materialwhich comprises the matrix. One embodiment of this invention comprisesinjecting the above-described aqueous acidizing compositions of thisinvention into a wellbore, the mixture being operative for slowlydissolving the more readily dissolvable siliceous material as foundadjacent to or in the pores of the subterranean formation. In apreferred embodiment of this invention the mixture is substantially freeof mineral acid other than hydrofluoric acid since a mineral acid is notrequired by this invention to produce hydrofluoric acid from thehexafluorotitanate-containing compounds by hydrolysis. However, it isnot necessary for the acidizing composition to be free of a mineral ororganic acid since the hexafluorotitanate-containing compound of thisinvention is operable for producing HF by hydrolysis whether or not anacid is present and reaction of the hexafluorotitanate with an acid(e.g. an acid other than HF) is not required to produce HF by hydrolysisof the hexafluorotitanate of this invention. The method furthercomprises allowing the mixture to flow deeply into the subterraneanformation away from the wellbore and allowing the mixture to react withthe siliceous material which is responsible for the low permeability ofthe formation and to slowly dissolve such siliceous material, therebyincreasing the permeability of the formation over a greater distancefrom the wellbore than the permeability of the more distant parts of theformation would be increased by acidizing with compositions havinghigher hydrofluoric acid concentration. It is to be understood that theincreased permeability is achieved by the slow in situ hydrolysis of thehexafluorotitanate-containing compound and by slowly producing a verylow concentration of hydrofluoric acid from thehexafluorotitanate-containing compound.

The method is especially useful for subterranean zones which arehydrocarbon producing zones such as oil and natural gas producing zones.

In one embodiment of this invention the formation is treated with aneffective amount of a sequestering or chelating agent to prevent theproduction of precipitates in the formation. Such treatments with asequestering agent can be as a separate step before and/or after thestep of acidizing the formation with the hexafluorotitanate-containingcompounds of this invention. Alternately the sequestering agent can becombined in the same step with the hexafluorotitanate-containingcompounds of this invention. Citrate sequestering agents are effectivefor preventing precipitation of reaction products, especially insolublealuminum salts and iron-containing compounds. The ammonium salt ofcitric acid is preferred over the sodium salt since the former lessensthe chance of insoluble sodium compounds from being formed. It should beunderstood, however, that the purpose of the sequestering agent is toprevent the production of detrimental precipitates in the formation andnot for reacting with the hexafluorotitanate-containing compounds ofthis invention for purposes of forming HF by said reaction.

In one embodiment the concentration of citrate ion is from about 0.01 toabout 1 molar. In another embodiment the concentrate of the citrate ionis from about 0.05 to about 0.5 molar and preferably from about 0.1 toabout 0.2 molar. In compositions containing citric acid, the pH of theacidizing composition may be as low as about 2.2 whereas if a citrate isused the pH of the acidizing composition may be as low as about 2.6.Although citric acid is usually cheaper than a citrate, and sodiumcitrate is usually cheaper than ammonium citrate, ammonium citrate ispreferred in very sensitive formations because it reduces the formationof sodium-containing precipitates and does not lower the pH of theacidizing composition as much as citric acid.

The method is particularly useful where the siliceous material in thesubterranean zone comprises clay. Non-limiting examples of clay mineralsfor which this method can be advantageously applied is attapulgite,bentonite, chlorite, halloysite, illite, kaolinite, montmorillonite, amixed-layer of the aforementioned clays, and various mixtures thereof.Furthermore, because of the very small concentration of hydrofluoricacid provided by this invention, the method can be employed informations having elevated temperatures without incurring commerciallysignificant damage to the formation. For example the acidizingcomposition and method of this invention can be used in siliceousmaterial having an elevated temperature between 65° C. and 125° C. orhigher without causing significant damage to the strength of thesiliceous material. This means that subterranean hydrocarbon-producingformations having elevated temperatures can be acidized using thecomposition and method of this invention without serious reduction inthe compressive strength of the treated formation. This inventiontherefore improves the permeability of a formation over greaterdistances from the wellbore than acidizing systems employing acidizingformulations having a pH of 2 or lower which can cause serious damage insome formations.

This invention is particularly useful in formations which are toosensitive for acidizing with other acidizing compositions.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graphical representation of the rate of dissolution ofbentonite in various acidizing compositions compared to the sodiumfluotitanate acidizing composition of this invention.

FIG. 2 is a graphical comparison of the rate of dissolution of clay fromsiliceous rock using 0.05M sodium fluotitanate where the rock isprereacted with a sequestering agent and where the reaction is conductedunder an inert atmosphere.

FIG. 3 is a graphical representation of depth of penetration of thereactive acidizing compositions of this invention as compared tohydrofluoric acid and fluoboric acid.

FIG. 4 is a diagram showing the permeability in a core sample after eachstep of a five step sequential treatment utilizing a 0.2M sodiumfluotitanate acidizing solution.

FIG. 5 is another diagram showing the permeability of a core samplewhich contained 7% CaCO₃ after each step of a five step sequentialtreatment utilizing a 0.1M ammonium fluotitanate acidizing solution.

FIG. 6 is a graph showing the permeability of a Somatito core sampleafter each step of a seven step sequential treatment utilizing a 0.1Mammonium fluotitanate acidizing solution.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Seven small ground samples of Wyoming bentonite, ground and screenedthrough no. 200 mesh, U.S. Sieve Series were reacted at 25° C. overseveral periods of time with the following aqueous acidizing solutions:

(1) 0.1M ammonium bifluoride (NH₄ HF₂)

(2) 0.1M hydrofluoric acid (HF)

(3) 0.1M sodium monofluorophosphate (Na₂ PO₂ F) with the solution pHadjusted to 4

(4) 0.05M ammonium fluoride (NH₄ F) - 0.05 M HF mixture

(5) 0.1M fluoboric acid (HBF₄)

(6) 0.1M Na₂ PO₂ F with the solution pH adjusted to 7

(7) 0.05M sodium fluotitanate (Na₂ TiF₆)

The amount of silicon dissolved as a function of reaction time for theseven experiments is shown in FIG. 1. The data presented in FIG. 1 showsthat the sodium fluotitanate solution dissolved bentonite much moreslowly than the other acidizing formulations.

A sample of the Wyoming bentonite was analyzed and found to have thefollowing cation content, expressed as %, ppm or ppb by weight of thetotal cation content in the oxide state,: 2.33% Na, 1.21% K, 1.06% Ca,1.80% Mg, 3.87% Fe, 18.21 Al, 66.90% Si, 0.263% Ti, 0.046% P, 0.076% Baand 0.020% Mn, and expressed as parts per million (ppm): 274 ppm Sr,<250 ppm V,

13 ppm Cr, 8 ppm Co, 11 ppm Ni, 8 ppm Cu, <50 ppm Mo, 26 ppm Pb, 62 ppmZn, <5 ppm Cd, <2 ppm Ag, <4 ppp Au, <25 ppm As, <30 ppm Sb, <100 ppmBi, <2500 ppm U, <50 ppm Te, <5 ppm Sn, <1200 ppm W, 23 ppm Li, 2.1 ppmBe, <400 ppm B, 125 ppm Zr, 39 ppm La, 65 ppm Ce, <150 ppm Th, andexpressed as parts per billion (ppb): 20 ppb Hg; which when totalledamounts to 95.788% of the cation content in the oxide state of theWyoming bentonite sample.

It has also been found that by treating certain types of siliceous rockwith a sequestering or chelating agent before acidizing with thehexafluorotitanate-containing acidizing solution greatly improves theamount of clay removed from the rock. FIG. 2 shows that a 0.05M sodiumfluotitanate acidizing solution of this invention dissolves more claysfrom Talara Zapotal rock when the rock is prereacted with disodiumethylenediamine tetraacetic acid (disodium EDTA) before the rock isreacted with the fluotitanate solution. This is an unexpected andsurprising result since it is not believed that pretreatment withsequestering agents improves the clay removing capabilities of otheracidizing compositions. Treatment of the rock withhexafluorotitanate-containing compositions under an inert atmosphere wasalso shown to increase the amount of silicon dissolved over thatexperience when acidizing in air, but the increase was not as great asthat experienced by pretreatment of the rock with EDTA which exhibitedsilica solubilization over 2.5 times the amount experienced when thecontrol sample was acidized in air.

The Talara Zapotal rock from the Talara, Peru, oil field formation wasassayed and found to contain by weight 2% Na, 4% Mg, 17% Al, 64% Si, 3%K, 2% Ca, 2% Ti and 6% Fe. A mineralogical analysis indicated that therock was by weight 44% plagioclase, 20% quartz, 16% feldspar, 8% illite,6% chlorite clay, 4% kaolinite and 2% analcime.

Accordingly one embodiment of this invention further comprises reactingthe siliceous material with a sequestering or chelating agent so thatacidizing with the hexafluorotitanate-containing acidizing compositionsof this invention will result in improved solubility of clay containedin the siliceous material. Reacting the siliceous material with asequestering agent can be carried out as a separate step prior to theacidizing step or combined with the acidizing step. In one embodimentthe sequestering agent is selected from the group consisting of citrateion-containing substances, EDTA and mixtures thereof. In a still furtherembodiment the siliceous material is reacted with a sequestering agentafter the acidizing step to prevent the precipitate of reaction productsproduced by the acidizing step thereby improving the stability of theacidized formation. In another embodiment of this invention thesiliceous material is subjected to an inert environment beforeacidizing. In a still further embodiment of this invention, asubterranean formation is first subjected to an inert gas, for examplenitrogen, prior to acidizing the formation. While not wishing to bebound by theory, it is believed that such inerting or sequesteringtreatments prevent the iron compounds of the siliceous material frombeing acidized by the acidizing compositions of this invention andthereby allow the acidizing composition to dissolve more clay.

Based on experimental solubilization data and a computer simulatedacidizing program, the extent of penetration of the reactive acidizingsolution from the wellbore into a Wyoming bentonite-containing formationwas calculated for 0.1M HF, 0.1M HBF₄ and 0.05M hexafluorotitanatecontaining acidizing solutions. The calculated results based upon themodel program disclosed in Chapter 9, section 9.2, pages 538 to 539 ofWELL DESIGN, DRILLING AND PRODUCTION, Prentice-Hall, Inc., which ishereby incorporated herein by reference, are shown in FIG. 3. As can beseen in FIG. 3 the penetration of reactive acidizing composition awayfrom the wellbore is far greater for hexafluorotitanate-containingcompositions than for HF and HBF₄ solutions. For example, the calculatedpenetration of the HF solution is only about 0.6 feet and the HBF₄solution about 1.2 feet while the sodium fluotitanate is over 11 feet.This calculation was based on the use of a 0.05M sodium fluotitanateacidizing solution conducted at 25° C. Clay stabilization agents canalso be used to stabilize ion exchange sites in the mineral matter orclay in the formation. The formation can be contacted with a claystabilization agent before, or after, or both before and aftercontacting the formation with the first substance orfluotitanate-containing aqueous composition. Preferably the claystabilization agent is an aqueous mixture containing a quaternary amineand ammonium chloride. Preferably the quaternary amine is a polymericquaternary amine. The clay stabilization agent attaches to the ionexchange sites near the surface of the clay particles and prevents theclays from swelling and preferably also forms bridges or linkages withthe clay thereby preventing clay particles from breaking off during theacidification step or subsequent oil producing step. The claystabilization agent keeps the clay in place and reduces formation damagecaused by clay swelling, fine particle migration and reprecipitation offine material.

Preferably ammonium chloride is used with both the clay stabilizationagent and the fluotitanate. The purpose of the ammonium chloride is toassist both the clay stabilization agent and the fluotitanate bymaintaining the salts which are formed in the solubilized state.Although sodium or potassium chloride can also be used with thestabilization agent or fluotitanate, ammonium chloride is preferredbecause of the higher solubility of ammonium salts over sodium orpotassium salts.

EXAMPLE 1

Low oil recovery from the Talara Field, Peru, was attributed to its verylow formation permeability which was determined to range from 0.1 to 5md with an average of 1 md. Conventional acid stimulation using 3%HF/12% HCl (mud acid) and 15% HCl resulted in failure. Accordingly theformation is considered a good candidate for stimulation using theacidizing compositions and method of this invention. Laboratory dataproduced from core flow experiments with Talara Zapotal coresdemonstrated about 300% permeability improvement as a result of anacidizing program utilizing this invention. The first test utilized anacidizing sequence consisting of the following five steps which arereferred to in FIG. 4 and described below. Examples of claystabilization agents useful in this process are water soluble quaternaryammonium salts such as tetramethylammonium chloride as disclosed in U.S.Pat. No. 3,797,574, or organic cationic or polycationic polymers orcopolymers such as quaternary polymers with nitrogen or phosphorous ortrivalent or tertiary sulfur such as disclosed in U.S. Pat. Nos.4,393,939, or 4,366,071, or 4,366,072, or 4,366,073, or 4,366,074, or4,374,739, or 4,447,342, or 4,366,073, or 4,462,718, or British PatentSpecification No. 1,590,345, or U.K. Patent Application GB No. 2,098,196A, or European Patent application publication No. 0,092,340, orpolycationic polymers and copolymer containing two or three quaternaryammonium moieties such as those disclosed in U.S. Pat. Nos. 4,497,596 or4,536,305, which aforementioned U.S. and foreign patents and foreignpatent applications are hereby incorporated herein by reference to anyextent deemed necessary for any purpose. Other examples of claystabilization agents useful in this process are water soluble organicacid salt or mixture of salts having the formula ##STR1## such as thosedisclosed in U.S. Pat. No. 4,536,297 or European Patent Applicationpublication No. 0,137,872, or aqueous chlorides such as ammoniumchloride as disclosed in U.S. Pat. No. 3,543,856, or inorganicpolycationic polymers such as zirconyl chloride or aluminum hydroxychloride, or water soluble alkali metal halides, alkaline earth metalhalides or ammonium halide, which aforementioned U.S. and foreign patentapplications are hereby incorporated herein by reference to any extentdeemed necessary for any purpose.

The clay stabilization agent used in the experiments represented byFIGS. 4, 5 and 6 is a polymeric quaternary amine stabilization agentsold by Halliburton Services under the trademark CLA-STA II. However itis to be understood that other clay stabilization agents can be usedwhich are operable for stabilizing clay and other fine particles in theformation to be treated. Preferably the clay stabilization agent ispolymeric structure that is absorbed on water sensitive clays as ionsconnected by a chain-like linkage structure. Such ion-linking claystabilization agents are preferred over agents which are absorbed oncation exchange sites as separate or single ions.

Initially the core of the experiment represented by FIG. 4 was heated to68° C. (154° F.) and maintained under a pressure of 35 kg/sq cm (550psig) and then subjected to the following stabilizing-acidizingsequential steps. First the core was treated with a quaternary aminestabilization agent, hereinafter referred to as "QASA,"-NH₄ Cl solution,or QASA/NH₄ Cl solution, to stabilize the clays and other fine particlecomponents of the core. The core was then acidized with a 5%hydrochloric acid solution which preferably contains a corrosioninhibitor, which was followed by restabilization with QASA/NH₄ Clsolution. Next the core was then treated with a 0.2M sodiumfluotitanate, 0.15M ammonium citrate or (NH₄)₂ HC₆ H₅ O₇ acidizingsolution at 68° C. (154 °F.) and 35 kg/sq. cm (500 psig), which wasfollowed by a third stabilization treatment with QASA/NH₄ Cl solution.The results as shown in FIG. 4 show an improvement in permeability fromabout 7.5 md after the first stabilization step to about 20 md after thethird stabilization step.

EXAMPLE 2

Another Talara Zapotal core was subjected to a similar five-stepsequential treatment as that shown in FIG. 4 except that the fourth steputilized a 0.1M ammonium fluotitanate/(NH₄)₂ HC₆ H₅ O acidizing solutioninstead of the sodium fluotitanate solution used in Example 1. Theresults, presented graphically in FIG. 5, show an improvement in corepermeability from about 75 md after the first stabilization step toabout 220 md after the third stabilization step.

EXAMPLE 3

The permeability of a core sample from Somatito Well 9109, having acomposition similar to that of the Wyoming bentonite described above,was improved from 0.03 millidarcy (md) to 1.62 md, more than 5000%, by aseven step sequential treatment of this invention conducted at 43° C.(110° F.) and 70 kg/sq cm (1000 psig). The results are shown graphicallyin FIG. 6. The composition of the treating solutions used for each stepand the permeability after each step were as follows:

    ______________________________________                                                                    Permeability                                      Step     Treating Solution  (md)                                              ______________________________________                                        (1)      QASA/NH.sub.4 Cl   0.03                                              (2)      5% HCl + 0.07% citric acid                                                                       0.07                                              (3)      QASA/NH.sub.4 Cl   0.09                                              (4)      0.1 M ammonium fluotitanate,                                                                     0.98                                                       0.15 M citric acid,                                                           0.2 M NH.sub.4 Cl                                                    (5)      QASA/NH.sub.4 Cl   1.10                                              (6)      5% HCl + 0.07% citric acid                                                                       1.43                                              (7)      QASA/NH.sub.4 Cl   1.62                                              ______________________________________                                    

EXAMPLE 4

A well having a lower than desired permeability from an isolated 9 m (30ft.) zone or interval is subjected to a five step sequential matrixacidization process similar to that shown in FIG. 5. The sequence andinjection rate are as follows:

    ______________________________________                                        Step      Injected Composition                                                ______________________________________                                        (1)       1018 1/m (82 gal/ft) QASA/NH.sub.4 Cl                                         (In a 30 ft. interval this rate corresponds                                   to an injection of 2460 gal.)                                       (2)       1018 1/m (82 gal/ft) 5% HCl                                         (3)       646 1/m (52 gal/ft) 0.1 M                                                     ammonium fluotitanate solution                                      (4)       5875 1/m (473 gal/ft) 5% HCl                                        (5)       1018 1/m (82 gal/ft) QASA/NH.sub.4 Cl                               ______________________________________                                    

An incremental improvement of about 5350 l/day (45 bbl/day) ispredicted.

EXAMPLE 5

A core from a conglomerate sandstone formation in the Mendoza ContractArea 7559 oil field in Argentina was treated in like manner to the corein Example 3 except for the following changes. The temperature wasraised to 104° (220° F.) and the pore pressure was held at 127 kg/sq cm(1800 psig) with a back pressure regulator, while an overburden pressureof 155 kg/sq cm (2200 psig) was maintained hydrostatically on the core.This test was conducted using the standard procedure of heat shrinking aTeflon (trademark) plastic sleeve over the entire cylindrical surface ofthe core so that the cylindrical surface of the core was sealed againstthe Teflon sleeve. The outside surface of the Teflon sleeve wassubjected to the overburden pressure of 155 kg/sq cm. The cylindricalends of the test core were therefore isolated from the overburdenpressure in the usual manner. With the test core in the verticalposition the injected solutions were forced to flow upwardly through thetest core. The discharge pressure of the solution flowing out of the topof the test core was regulated to a pressure of 127 kg/sq cm. Theinitial permeability of the core was 2.23 md. The core was then treatedwith the same sequence of fluids as that of Example 3. After the finaltreatment, the permeability had increased to 9.11 md or about 4 foldover the initial permeability. No unconsolidation or plugging of thecore occurred from the treatment by the principals of this invention.

In contrast to the excellent results described above, a similar corefrom the same sandstone formation treated with conventional mud acid,i.e. 3% HF/12% HCl, fell apart.

The permeability after each step, the steps are set forth in Example 3,was as follows:

    ______________________________________                                        Step       Permeability (md)                                                  ______________________________________                                        (1)        2.23                                                               (2)        1.95                                                               (3)        1.95                                                               (4)        4.11                                                               (5)        4.11                                                               (6)        9.11                                                               (7)        9.11                                                               ______________________________________                                    

EXAMPLE 6

During experiments with treating sandstone cores with solutions of puresodium fluotitanate, it was found that the aluminum concentration of thefiltrate was quite low. To determine if this was due to theprecipitation of aluminum-containing compounds such as aluminumfluoride-containing compounds, the following experiments were conducted.A sample of pure chlorite clay was ground to a powder.

Test A: Into a dish at room temperature was added 10 ml of a solutioncontaining 0.45M sodium fluotitanate, 0.156M sodium citrate, 0.1Mammonium chloride, and 15.4 mg of the powdered chlorite clay. This wasallowed to stand for 24 hours without agitation.

Test B: For comparison, 150 mg chlorite clay and 10 ml of 6M HCl werealso placed in a dish and allowed to stand in the same manner as in TestA.

The mixtures from Tests A and B were filtered and the analysis of thefiltrates was as follows:

    ______________________________________                                        Elemental Concentrations (mg/l)                                                                        Mole Ratio                                           Test Al      Si     Fe   Ca    Mn   Mg   Si/Al                                ______________________________________                                        A    14.9    39.9   21.2 69.5  <12  <10  2.58                                 B     3.8    12.1   15.2 8.55  <12  <10  3.07                                 ______________________________________                                    

Not all the chlorite dissolved in either Test A or B. The solidsremaining after treating were examined with x-ray diffraction and showedevidence for only the starting materials, chlorite clay, and in the caseof Test A sodium fluotitanate resulting from the evaporation of thesolution after filtering.

No precipitation of aluminum-containing compounds, or any othercompounds, was observed in Test A. These tests show that the citrate ioneffectively held aluminum ions in the fluotitanate solution therebypreventing precipitation of reaction products which can reduce thepermeability of formations when acidized.

In one embodiment the formulation of the clay stabilizationagent/ammonium chloride solution or the QASA/NH₄ Cl solution is fromabout 0.005 to about 2% by volume clay stabilization agent or QASA andfrom about 0.5 to about 4% by weight ammonium chloride. The percentageof stabilizer and ammonium chloride will vary depending on the amount ofclay in the formation to be treated. Various formulations and uses ofQASA is described in Halliburton's publication F-3183 (Revised) whichhereby is incorporated herein by reference.

Unless otherwise specified, the preferred formulation of the QASA/NH₄ Clsolution referred to above is as follows:

0.75% by volume QASA

2% by weight NH₄ Cl

In each of the above examples it is preferable that the varioushydrochloric acid solutions contain a corrosion inhibitor to protect thepiping and tubing of the well and injection system. Any commerciallyavailable corrosion inhibitors can be used for this purpose by mixingsuch inhibitors with HCl containing solutions. Examples of suchcorrosion inhibitors are Halliburton's inhibitors sold under thetrademarks "HAI50," "HAI65" and "HAI75" which are described

in Halliburton's publications CS-5073 and CS-5136 which are incorporatedherein by reference and Dowell's inhibitor sold under the trademark A200which is described in the Dowell Stimulation Materials Manual,acidizing-Sec. I-D-1, pages 1 and 2, December, 1981, which isincorporated herein by reference.

Unless otherwise specified, all percentages referred to are by weight.

It is understood that the foregoing detailed description and embodimentsshown in the Figures and presented in the Examples are illustrative ofthe principles of this invention. Other alternatives can be employed.For example, other sequential injection steps can be employed andvarious other fluids can be used for specific reasons such asstabilizers, removing skin damage adjacent the wellbore, dissolvingcalcareous materials, causing viscosity increase or thickening,inhibiting corrosion, preventing sludge formation and/or reducingfriction. Accordingly the present invention is not limited to that shownand described in the Examples and Figures.

What is claimed is:
 1. A process for slowly dissolving siliceousmaterial comprising:(a) providing an aqueous composition comprisingi.water, and ii. a first substance selected from the group consisting ofhexafluorotitanate-containing compounds, and mixtures thereof which areoperable for producing, without the presence of an acid in said aqueouscomposition other than hydrofluoric acid, hydrofluoric acid which isgenerated by reaction of hexafluorotitanate anions with said water,wherein the concentration of said first substance in said aqueouscomposition is from about 0.0001 molar to the solubility limit of saidfirst substance in said water; and (b) contacting siliceous materialwith an effective amount of said aqueous composition to slowly dissolvesaid siliceous material.
 2. The process of claim 1, wherein saidsiliceous material is contained in a subterranean formation.
 3. Theprocess of claim 1, wherein said siliceous material is clay mineralmatter in a subterranean formation.
 4. The process of claim 1, whereinsaid first substance is selected from the group consisting of ammoniumfluotitanate, sodium fluotitanate, and mixtures thereof.
 5. The processof claim 1, wherein the concentration of said first substance used toform said aqueous composition is from about 0.01 to about 1.3 molar. 6.The process of claim 1, wherein the concentration of said firstsubstance used to form said aqueous composition is from about 0.05 toabout 0.75 molar.
 7. The process of claim 1, wherein the pH of saidaqueous solution is at least about 2.2.
 8. The process of claim 1,wherein the pH of said aqueous solution is from about 2.2 to about 3.8.9. The process of claim 1, wherein said aqueous composition provided instep (a) also comprises from about 0.01 to about 1 molar concentrationof citrate ion.
 10. The process of claim 1, wherein the concentration ofsaid first substance used to form said aqueous composition is from about0.05 to about 0.75 molar.
 11. The process of claim 1, wherein theconcentration of said first substance used to form said aqueouscomposition is from about 0.1 to about 0.5 molar.
 12. The process ofclaim 1, wherein said aqueous composition provided in step (a) alsocomprises from about 0.05 to about 0.5 molar concentration of citrateion.
 13. The process of claim 1, wherein said aqueous compositionprovided in step (a) also comprises from about 0.01 to about 0.2 molarconcentration of citrate ion.
 14. The process of claim 1, furthercomprising contacting said siliceous material with an aqueous solutioncomprising a sequestering agent prior to contacting said siliceousmaterial with said aqueous composition in step (b).
 15. A process forincreasing the permeability of siliceous material having a claycomponent comprising:(a) contacting said siliceous material with a claystabilization agent/ammonium chloride aqueous mixture, said claystabilization agent being operative to stabilize mineral mattercontaining ion exchange sites; (b) after said contacting with said claystabilization agent in step (a), contacting said siliceous material witha hydrochloric acid solution having a HCl concentration about 2 to about5%; (c) after said contacting with said hydrochloric acid solution instep (b), contacting said siliceous material with a clay stabilizationagent/ammonium chloride aqueous mixture, said clay stabilization agentbeing operative to stabilize mineral matter containing ion exchangesites; (d) after said contacting with a clay stabilization agent in step(c), contacting said siliceous material with an effective amount of anaqueous composition comprising (i.) water, and (ii.) a first substanceselected from the group consisting of hexafluorotitanate-containingcompounds, and mixtures thereof which are operable for producing,without the presence of an acid in said aqueous composition other thanhydrofluoric acid, hydrofluoric acid which is generated by reaction ofhexafluorotitanate anions with said water, wherein the concentration ofsaid first substance in said aqueous composition is from about 0.0001molar to the solubility limit of said first substance in said water, toslowly dissolve a part of said siliceous material and increase thepermeability thereof.
 16. The process of claim 15, furthercomprising:(e) after said contacting with said aqueous composition instep (d), contacting said siliceous material with a clay stabilizationagent/ammonium chloride aqueous mixture, said clay stabilization agentbeing operative to stabilize mineral matter containing ion exchangesites.
 17. The process of claim 15, wherein said hydrochloric acidsolution used to contact said siliceous material in step (b) alsocomprises a citrate ion concentration of from about 0.001 molar to about0.05 molar.
 18. The process of claim 15, wherein the concentration ofsaid first substance in said aqueous composition of step (d) is fromabout 0.01 to about 1.3 molar.
 19. The process of claim 15, wherein saidaqueous composition of step (d) also comprises from about 0.01 molar toabout 1 molar concentration of citrate ion.
 20. A process for increasingthe permeability of a subterranean formation having siliceous clay insaid formation which causes said formation to have an initialpermeability lower than desired, said process comprising:(a) providingan aqueous acidizing composition which slowly produces hydrofluoric acidcomprisingi. water, and ii. a first substance selected from the groupconsisting of ammonium fluotitanate, sodium fluotitanate, and mixturesthereof, which are operable for slowly producing, without the presenceof an acid in said aqueous acidizing composition other than hydrofluoricacid, hydrofluoric acid which is generated by slowly hydrolyzinghexafluorotitanate anions formed from said first substance in saidwater, wherein the concentration of said first substance in said aqueouscomposition is from about 0.0001 molar to the solubility limit of saidfirst substance in said water; and (b) contacting said subterraneanformation with an effective amount of said aqueous acidizing compositionto slow dissolve siliceous clay in said formation to increase thepermeability of said formation to a value greater than said initialpermeability.
 21. The process of claim 20 wherein said aqueous acidizingcomposition provided in step (a) also comprises from about 0.01 to about1 molar concentration of citrate ion.
 22. A process for increasing thepermeability of a subterranean formation having siliceous clay in saidformation which causes said formation to have an initial permeabilitylower than desired, said process comprising:(a) providing an aqueousacidizing composition which slowly produces hydrofluoric acidcomprisingi. water, ii. a first substance selected from the groupconsisting of ammonium fluotitanate, sodium fluotitanate, and mixturesthereof, which are operable for slowly producing, without the presenceof an acid in said aqueous acidizing composition other than hydrofluoricacid, hydrofluoric acid which is generated by the slow reaction ofhexafluorotitanate anions formed from said first substance with saidwater, wherein the concentration of said first substance in said aqueouscomposition is from about 0.01 to about 1.3 molar, and iii. a citrateion concentration of from about 0.01 to about 1 molar; and (b)contacting said subterranean formation with an effective amount of saidaqueous acidizing composition to sowly dissolve siliceous clay in saidformation to increase the permeability of said formation to a valuesubstantially greater than said initial permeability.
 23. The process ofclaim 22, wherein said citrate ion concentration is formed from a secondsubstance selected from the group consisting of ammonium citrate, sodiumcitrate, citric acid and mixtures thereof.
 24. The process of claim 22,wherein said subterranean formation comprises chlorite clays.
 25. Anacidizing composition comprising:i. water, ii. a first substanceselected from the group consisting of hexafluorotitanate-containingcompounds,and mixtures thereof which are operable for producing, withoutthe presence of an acid in said aqueous composition other thanhydrofluoric acid, hydrofluoric acid generated by reaction ofhexafluorotitanate anions with said water, wherein the concentration ofsaid first substance in said aqueous composition is from about 0.0001molar to the solubility limit of said first substance in said water, andiii. from about 0.01 to about 1 molar concentration of citrate ion. 26.An acidizing composition comprising:i. water, ii. a first substanceselected from the group consisting of ammonium fluotitanate, sodiumfluotitanate, and mixtures thereof which are operable for producing,without the presence of an acid in said aqueous composition other thanhydrofluoric acid, hydrofluoric acid generated by reaction ofhexafluorotitanate anions with said water, wherein the concentration ofsaid first substance in said aqueous composition is from about 0.0001molar to the solubility limit of said first substance in said water, andiii. a second substance selected from the group consisting of ammoniumcitrate, sodium citrate, citric acid and mixtures thereof, wherein theconcentration of said second substance is from about 0.01 to about 1molar.
 27. The composition of claim 26, wherein the concentration ofsaid first substance is from about 0.01 to about 1.3 molar, and theconcentration of said second substance is from about 0.05 to about 0.5molar.